The inventive subject matter described herein relates generally to pneumatic braking systems and more particularly to a pneumatic braking system for a rail vehicle (e.g., a train consist) comprising a lead powered unit (e.g., a locomotive) and one or more remote powered units (e.g., remote locomotives).
One of the most critical aspects of the operation of railroad vehicles is the predictable and successful operation of the air brake system. However the air brake system is subjected to a variety of dynamic effects, not only as a result of the controlled application and release of the brakes in response to changes in brake pipe pressure, but also due to the varying operating conditions encountered by the train. Thus multiple operating scenarios must be considered for the successful design and operation of the air brake system.
At each railcar, a control valve (typically comprising a plurality of valves and interconnecting piping) responds to locomotive operator-initiated changes in the brake pipe fluid pressure by applying the brakes (in response to a decrease in the brake pipe fluid pressure) or by releasing the brakes (in response to an increase in the brake pipe fluid pressure). The fluid within the brake pipe conventionally comprises pressurized air. The control valve at each railcar senses the drop in brake pipe air pressure as the pressure drop propagates along the brake pipe. In response, at each railcar pressurized air is supplied from a local railcar reservoir to the wheel brake cylinders, which in turn drive the brake shoes against the railcar wheels. The railcar reservoir is charged by supplying air from the brake pipe during non-braking intervals. Typically, the pressure reduction in the brake pipe for signaling a brake application is about seven to twenty-six psi, with a nominal steady state brake pipe pressure of about 90 psi. The braking pressure applied to the railcar wheels is proportional to the drop in the brake pipe pressure. Thus it can be seen that the brake pipe serves to both supply pressurized air to each railcar for powering the brake shoes during a brake application and also serves as the medium for communicating brake application and release instructions to each railcar.
The railcar brakes can be applied in two different modes, i.e., a service brake application or an emergency brake application. A service brake application involves the application of braking forces to the railcar to slow the train or bring it to a stop at a forward location along the track. During service brake applications the brake pipe pressure is slowly reduced and the brakes are applied gradually in response thereto. An emergency brake application commands an immediate application of the railcar brakes through an immediate evacuation or venting of the brake pipe. Unfortunately, because the brake pipe runs for several thousand yards along the length of the train, the emergency braking evacuation does not occur instantaneously along the entire length of the brake pipe. Thus the braking forces are not uniformly applied at each railcar to stop the train.
After one emergency brake application or two or three service brake applications, the brake pipe must be recharged to its nominal operating pressure by supplying pressurized air from a reservoir on the locomotive into the brake pipe. Effective subsequent brake applications cannot be made until the recharging process has been completed.
FIG. 1 illustrates a typical prior art brake system employed by a railway freight train. In a conventional train having only a lead locomotive, the train brake system comprises a locomotive brake system located on a locomotive 100 and a set of railcar brake systems located on a plurality of railcars illustrated by a railcar 200. The application and release of braking action is controlled by an operator within the locomotive 100, who uses a manually operated brake handle to effect braking action. The locomotive includes an air brake control system 102 for supplying air pressure to or controllably venting a pressurized brake pipe 101 via a relay valve 117. The pressurized brake pipe 101 is in fluid communication with each of the railcars 200 of the train as shown.
The locomotive brake control system 102 comprises an air supply input link 111 for supplying pressurized fluid (air) through which the brake pipe 101 is charged. A flow measuring adapter 113 is connected to the air supply link 111 for measuring the charging rate (as a differential pressure between the air supply and output port 116) of the brake control system 102. The output port 116 of the flow measuring adapter 113 is connected to an input port 121 of a relay valve 117. A bi-directional port 122 of the relay valve 117 is coupled to the brake pipe 101. The relay valve 117 further includes a port 123 coupled through an air pressure control link 103 to an equalizing reservoir 105. The pressure control link 103 is also connected to a pressure control valve 107 through which the equalizing reservoir 105 is charged and discharged in the process of a brake operation. A port 124 of the relay valve 117 is controllably vented to the atmosphere as an exhaust port. Coupled with brake pipe 101 and air pressure control link 103 are respective pressure measuring and display devices 131 and 133. The brake pipe gauge 131 measures the air pressure in the brake pipe 101 and the equalizing reservoir gauge 133 measures the pressure in the equalizing reservoir 105.
The components of a railcar air brake control system 202, include a control valve 203 having a port 221 coupled to the brake pipe 101. The control valve 203 also includes a port 222 coupled to a pressure storage and reference reservoir 205. Finally, the control valve 203 includes a port 223 coupled to an air brake cylinder 231, comprising a piston 232 connected to a brake shoe 233. An increase in air pressure at the port 223 is fluidly communicated to the piston 232 for driving the brake shoe 233 against the wheels 235 of the railcar 200. Thus the air brake control system 102 of the locomotive 100 controls operation of the pneumatically operated brake shoes 233 at each of the wheels 235 of each railcar 200.
During train operation, the brake pipe valve 120, through which the components of the brake control system 102 are coupled to the brake pipe 101, is open to create a continuous brake pipe fluid path between the locomotive 100 and all of the railcars 200 of the train. The brake pipe valve 120 is controlled by a brake valve cut-out valve 250, that is in turn, controlled by a pilot valve 251. The pilot valve 251 can be manually operated by the locomotive operator to close the brake pipe valve 120 when it is desired to terminate brake pipe charging. There are also other valves and control components (not shown in FIG. 1) that automatically terminate brake pipe charging during an emergency brake application by activating the pilot valve 251 which closes the brake pipe valve 120. Each railcar 200 also includes a manually-operated brake pipe valve 240, as shown in FIG. 1.
The brake system is initially pressurized by the operation of the pressure control valve 107, which controls the air supply to the control link 103 to charge the equalizing reservoir 105 to a predetermined pressure. The relay valve 117 is then operated to couple port 121 with the port 122 so that air is supplied there through to the brake pipe 101, charging the brake pipe 101 to the predetermined charge pressure, as established by the pressure of the equalizing reservoir 105. When the brake pipe pressure reaches the predetermined pressure, the pressure at the port 122 (connected to the brake pipe 101) equals the pressure at the port 123 (connected to the equalizing reservoir 105). This condition indicates a charged brake pipe and the fluid flow path from the air supply port 121 to the brake pipe 101 via the relay valve 117 is closed.
The pressure storage and reference reservoir 205 of each railcar 200 is fully charged from the brake pipe 101 through the control valve 203, thereby establishing a reference pressure for maximum withdrawal of the piston 232 and complete release of the brakes 233 for each of the railcars 200.
To brake the railcars 200, the train operator operates the pressure control valve 107 using the braking handle in the locomotive cab. This operation causes a partial venting of the air pressure control link 103 through the exhaust port of the pressure control valve 107, reducing the pressure within the equalizing reservoir 105. This pressure reduction is sensed by the relay valve 117 at the port 123. In turn, the pressure reduction causes the bi-directional port 122 to be coupled to the exhaust port 124, thereby exhausting the brake pipe 101 to the atmosphere. The venting of the brake pipe 101 continues until the pressure within the brake pipe 101 equals the pressure of equalizing reservoir 105.
As the pressure in the brake pipe 101 falls, the control valve 203 in each of the cars 200 senses the pressure reduction by comparing the brake pipe pressure with the pressure storage and reference reservoir pressure. This pressure reduction causes a corresponding increase in the air pressure applied to the brake cylinder 231 from the port 223, resulting in an application of the brake shoes 233 against the wheels 235 in proportion to the sensed pressure reduction in the brake pipe 101.
Further pressure reductions in the equalizing reservoir 105 by the train operator produce corresponding pressure reductions in the brake pipe 101 and, corresponding additional braking effort by the brake shoes 233 in each of the railcars 200. In summary, the intended operation of the brake system in the cars 200 and specifically the braking effort applied in each of the cars 200, is proportional to the reduction in pressure in the equalizing reservoir 105 within the locomotive 100.
When the locomotive operator desires to release the train car brakes, he/she operates the pressure control valve 107 via the braking handle, to effectuate a recharging of the air brake system 102. The recharging is accomplished by bringing the pressure within the equalizing reservoir 105 back to its fully charged state by supplying pressurized air via the air supply. With the equalizing reservoir 105 recharged, there is again a pressure differential (but opposite in sign to the previous pressure drop in the pressure line 103) between the ports 122 and 123 of the relay valve 117 that causes the brake pipe 101 to be charged with pressurized air from air supply 111 through the flow measuring adapter 113 and the relay valve 117. The brake pipe pressure increase is sensed by the control valve 203 in each of the railcars 200 to cause the brake shoes 233 to be released by the action of the brake cylinder 231.
Distributed power train operation supplies motive power from a lead locomotive and one or more remote locomotives spaced apart from the lead unit in the train consist. Distributed train operation may be preferable for long train consists to improve train handling and performance. Each lead and remote locomotive includes an air brake control system, such as the air brake control system 102 discussed above, and a communications system for exchanging information between the lead and the remote units. Conventionally the communications system comprises a radio frequency link and the necessary receiving and transmitting equipment at each of the lead and the remote units.
The description of the inventive subject matter below with respect to the brake control system of a remote locomotive in a distributed power train consist refers to the same brake control system components and uses the same reference characters as described above in conjunction with the brake control system of the lead locomotive. Specific mention will be made if the reference pertains only to the lead or only to the remote locomotive.
On distributed power trains equipped with UIC (Union Internationale de Chemins Fer) wagon braking equipment, braking is accomplished by venting the brake pipe 101 at both the lead and remote locomotives, thus accelerating the brake pipe venting and the application of the brakes at each railcar, especially for those railcars near the end of the train. Brake pipe venting at only the lead unit requires propagation of the brake pipe pressure reduction along the length of the train, thus slowing brake applications at railcars distant from the lead unit. For a distributed power train with an operative communications link between the lead and remote units, when the train operator commands a brake application by operation of the brake handle at the lead unit, a brake application command is transmitted to each remote unit over the radio frequency communications link. In response, each remote unit also vents the brake pipe through its respective relay valve 117. Thus braking actions at the remote locomotives follow the braking actions of the lead unit in response to signals transmitted by the communications system. As a result, the entire brake pipe is vented faster than if the venting occurred only at the lead locomotive. A brake release initiated at the lead unit is also communicated over the radio frequency link to the remote units so that the brake pipe 101 is recharged from all locomotives.
If the communications system is inoperative or if the communications link between the lead unit and the remote units is disrupted (for example, if line-of-sight directivity is lost due to track topology or an interfering object), when the lead operator makes a brake application the remote locomotives will not receive the brake application command via the communications system. Thus the brake application is executed by venting the brake pipe only at the lead locomotive resulting in a slower brake application at all the railcars.
It is known that leaks can develop in the brake pipe, causing unwanted pressure reductions. Thus in one operational mode for a distributed power train, the remote units (and the lead unit) continually charge the brake pipe 101 when the pressure falls below a nominal value (i.e., whenever a brake application is not in progress). A remote unit senses the brake pipe pressure via the relay valve 117, that compares the equalizing reservoir pressure with the brake pipe pressure. Whenever the brake pipe pressure is less than the equalizing reservoir pressure, the brake pipe 101 is charged from the air supply 111 via the relay valve 117 of the remote unit. However, a remote unit should not recharge the brake pipe when a brake application has been initiated at the lead unit.
A dangerous scenario can develop if a brake application command transmitted over the communications link from the lead unit does not reach the remote locomotive while the latter is monitoring and recharging the brake pipe to compensate for pressure reductions caused by leaks within the brake pipe 101. Typically, the recharging process is initiated if the brake pipe pressure falls below a nominal predetermined value. In this situation the remote locomotive continues to recharge the brake pipe 101 as the lead unit is venting the brake pipe to signal a brake application to the railcars 200. This situation can cause dangerously high in-train forces to develop.
One prior art technique for avoiding this scenario is to automatically close the brake pipe valve 120 of the remote unit whenever communications is lost between the lead and the remote locomotive units. With the brake pipe valve 120 closed, the remote units cannot recharge (nor vent) the brake pipe 101. Thus all brake signaling (both brake applications and brake releases) over the brake pipe 101 is initiated from the lead unit. Although under this condition the remote locomotives cannot assist with the brake pipe venting to accelerate brake applications at the railcars 200, the remote locomotives also cannot erroneously recharge the brake pipe while the lead unit is venting it.
The prior art LOCOTROL® distributed power communications system (available from the General Electric Company of Schenectady, N.Y.) incorporates a variant of the technique described above by including a brake pipe flow sensing function at each remote locomotive in a distributed power train. A flow sensor, such as the flow measuring adapter 113 as depicted in FIG. 1, is included in the brake pipe charging path at each remote unit to detect air flow from the air supply through the relay valve 117 to the brake pipe 101. If the flow rate (which is determined by a differential pressure) exceeds a predetermined value, a brake application is declared. That is, the brake pipe pressure has fallen to a value consistent with a brake application (which would have been initiated from the lead locomotive). If concurrently the communications system is inoperative, then in response to the simultaneous occurrence of these two events, the remote unit brake pipe valve 120 is commanded to a cut-out or closed position. Proper execution of the command closes the remote unit brake pipe valve 120. As a result, the brake application initiated by the venting of the brake pipe at the lead unit cannot be countered by pressurizing of the brake pipe at the remote unit.
If the command to cut-out or close the brake pipe valve 120 is not properly executed, then the brake valve at the remote unit remains open. There are several possible causes for this scenario, including a failure of the brake valve cut-out valve 250 (i.e., the valve that drives the brake pipe valve into a cut-off or closed configuration), a failure of the pilot valve 251 that drives the brake valve cut-out valve, or a brake pipe valve 120 stuck in the open position. Thus, if the brake pipe valve is not closed or cut-out as commanded, and during a communications system failure the lead unit issues a brake application, then the remote units continue to supply brake pipe recharging pressure while the lead unit is venting the brake pipe to apply the railcar brakes. This sets up an undesirable situation where the front railcars experience maximum braking and rear railcars experience minimum or no braking action. The net result is that the rear of the train can run into the front of the train, causing high in-train forces and possible derailment.
In very long conventional trains when the operator makes a brake application, the brake pipe pressure must be exhausted from the front or leading locomotive. Since the brake pipe length is very long, the front part of the train will be heavily applying the brakes while the rear part of the train may still be reducing its brake pipe and the resulting car braking reduced. This situation sets up a similar undesirable situation as above where the front part of the train has full braking and the rear part of the train has minimal braking resulting in potentially high in-train forces, which could possibly cause a derailment.
The situation above becomes more pronounced in distributed power trains where train lengths are much longer than conventional trains. With good communications between the lead and remote, train braking is applied simultaneously at both the front of the train via the lead locomotive and the rear part of the train via the remote locomotive. This provides an even reduction in brake pipe pressure throughout the train, which results in a more uniform braking effort by the cars resulting in lower in-train forces. When communications are disrupted in distributed power train, and the operator makes a brake application, then the undesirable situation of heavy braking at the front of the train and minimal braking at the rear of the train still occurs. Even with the remote locomotive detecting the brake application and cutting out the brake valve, the brake pipe pressure must still be exhausted by the lead locomotive which results in the front part of the train having full braking and the rear part of the train having minimal braking.